Liquid crystal displays (LCDs) and other display devices, as well as touch panels and other input devices to be used in combination with the display devices have been recently widely employed in various areas. Transparent pressure-sensitive adhesive sheets (pressure-sensitive adhesive tapes) are used for the lamination of optical members typically in the production of such display devices and input devices. For example, transparent pressure-sensitive adhesive sheets are used for the lamination of touch panels with various display devices or optical members (e.g., protective plates) and for the lamination of constitutional members of touch panels with each other (see, for example, Patent Literature (PTL) 1, PTL 2, PTL 3, and PTL 4).
Pressure-sensitive adhesive sheets to be used in these devices should exhibit not only transparency in an ordinary state but also sufficient properties as pressure-sensitive adhesive sheets in a variety of environments with expanding uses of the display devices and input devices. Typically, they should have a property as not to become cloudy (whitened) due to moisture (high humidity) (this property is hereinafter also referred to as “whitening resistance”) and, in particular, should be resistant to whitening even when the environment is changed from a high temperature and high humidity environment (hot and humid environment) to a room-temperature environment. They also require bonding reliability at high temperatures (high-temperature bonding reliability). Specifically, they require excellent tackiness at high temperatures and such a property as not suffer from blistering and separation at high temperatures (hereinafter this property is also referred to as “blistering/separation resistance”).
There is known a pressure-sensitive adhesive having high transparency and excellent whitening resistance (see PTL 5). However, a pressure-sensitive adhesive sheet formed from the pressure-sensitive adhesive, when exposed to high temperatures, has an insufficient adhesive strength and suffers from blistering and separation, thus being inferior in high-temperature bonding reliability. Specifically, under present circumstances, a pressure-sensitive adhesive sheet having both whitening resistance and high-temperature bonding reliability at certain level has not yet been obtained.
Of the display devices and input devices, those including members having steps or bumps such as printed-ink bumps have increased in number. For example, cellular phones employ touch panels having members with frame-like printed regions. Pressure-sensitive adhesive sheets to be used in such applications require not only a capability of affixing members but also a capability of filling in printed-ink bumps, namely, excellent bump absorptivity (bump conformability).